Mine is a beast. It's got one of those monstrous PC P&C TurboCool 1KW PSUs in it that's full ATX spec (much longer and heavier than "standard"). The steel case is frickin' heavy and the Thermalright Ultra-120 Extreme is one massive heatsink too. Additionally, there are two optical drives, six 5-platter HDDs, an external SATA bay with a seventh HDD, GTX280, 9800GTX+, and every card slot occupied (Tri-SLI board).
Here's how I know I'm in the top category without even weighing it: I attached one of those Antec LANboard roller things with the GearGrip-like handle and the snaps on the straps pop off when I try to pick it up. They can't handle the weight. I still leave it attached to roll it around, so that's even more weight.